1. Field of the Invention
This invention relates to an optical recording and reproducing apparatus, and in particular to an optical recording and reproducing apparatus using a separate-type optical head which is divided into a stationary optical system and a movable optical system.
2. Description of the Related Art
FIG. 5 shows a conventional optical recording and reproducing apparatus which is disclosed in Japanese Laid-Open Patent Application No. 62-95743. The illustrated apparatus comprises a stationary optical system 20 and a movable optical system 24. The stationary optical system 20 includes a semiconductor laser 2, a polarizing beam splitter 4, a quarter-wavelength plate 5, a half mirror 10, a convex lens 11, a cylindrical lens 12, a knife edge 13, and split-beam detectors 14 and 15. The movable optical system 24 includes a lens carriage 21 on which a launching mirror 6 and an objective lens 7 are mounted, an electromagnetic driving means 22, and a rail 23, the lens carriage 21 being moved along the rail 23 by the electromagnetic driving means 22.
Next, the operation of this conventional optical recording and reproducing apparatus will be described. First, the semiconductor laser 2 is driven by means of a power source 1. The semiconductor laser 2 then emits a laser beam, which is converted to a parallel beam by a collimating lens 3. The parallel beam is then transmitted as P-polarized light through the polarizing beam splitter 4, the quarter-wavelength plate 5, and the launching mirror 6 before it reaches the objective lens 7. The beam is then condensed on a disk 8 as a concentrated spot 9 having a diameter of about 1 .mu.m. The beam is then reflected by the disk 8 and is transmitted through the objective lens 7 to become a parallel beam. This parallel beam is reflected by the launching mirror 6 before being transmitted through the quarter-wavelength plate 5. As a result of being transmitted both ways through the quarter-wavelength plate 5, the beam is rotated to S-polarized light and impinges upon the polarizing beam splitter 4, where it is reflected to the half mirror 10. The half mirror 10 divides the beam into two different beams. One of the beams is reflected by the half mirror 10 to a focal-deviation detecting means which is composed of the convex lens 11, the cylindrical lens 12, the knife edge 13, and the split-beam detector 14. Since the principles of operation of this focal-deviation detecting means are not directly related to this invention, a detailed explanation there of will be omitted. It should be added, however, that this focal deviation detecting means may be based on a well-known focus detecting method such as the knife-edge method, image rotation method, Foucault's method, or astigmatism method.
The output of this split-beam detector 14 is converted to a focal-deviation signal by a calculation circuit (not shown). An actuator (not shown) moves the objective lens 7 in the direction of the optical axis, controlling the condensed spot 9 in such a manner that it is constantly kept in focus on the disk 8.
The other beam, which is transmitted through the half mirror 10, is received by the split-beam detector 15, causing a differential circuit 16 to output a track-deviation signal 17. This signal is supplied to the electromagnetic driving means 22, which employs, for example, a well-known voice coil. The electromagnetic driving means 22 then moves the lens carriage 21 along the rail 23 in the radial direction of the disk 8, thereby effecting tracking control.
Detection of track deviation will now be described in more detail. If, as shown in FIG. 6A or FIG. 6C, the condensed spot 9 is positioned in the middle of a guide groove (track) 8a or in the middle of an inter-groove section, the quantity of light incident on a light receiving surface 15a is the same as that incident on a light receiving surface 15b, as shown in FIG. 6E or FIG. 6G. However, when the condensed spot 9 deviates to one side of a guide groove, as shown in FIG. 6B, the quantity of light incident on the light receiving surface 15a (indicated by the shaded portion) is less than that incident on the light receiving surface 15b because of the diffraction attributable to the guide groove 8a, as shown in FIG. 6F. When the condensed spot 9 deviates to the other side of a guide groove, as shown in FIG. 6D, the quantity of light incident on the light receiving surface 15b (indicated by the shaded portion) is less than that incident on the light receiving surface 15a, as shown in FIG. 6H. Accordingly, on the basis of a difference in output between the two light receiving surfaces 15a and 15b, it can be detected whether or not the condensed spot 9 is correctly positioned with respect to a guide groove 8a as well as to which side of the guide groove 8a it has deviated. This tracking detection method is generally called the diffraction light method or push-pull method.
However, conventional optical recording and reproducing apparatuses as described above have the following problem. When the movable optical system 24 has been displaced in the vertical direction or when the axis of movement of the rail 23 is not exactly parallel to the optical axis of the parallel beam output from the stationary optical system 20, an offset is generated in the track-deviation signal as the movable optical system 24 moves along the rail 23.
A signal offset is also easily generated if the movable optical system 24 is displaced during its movement due to dust or the like adhering to the rail 23.
This problem will now be discussed with reference to FIGS. 7, 8A, 8B, 9A and 9B. First, when in FIG. 7 the movable optical system 24 is in its initial position indicated by the dashed line, the launching mirror 6 is also in its initial position indicated by the solid line. In this state, the beam incident on the split-beam detector 15 can be indicated by the solid lines. FIG. 8A shows the manner in which beams are received by the split-beam detector 15. The track-deviation detecting signal 17 obtained from the split-beam detector 15 is then set by initialization in such a manner that it exhibits no offset, as shown in FIG. 9A.
Next, suppose, in FIG. 7, the movable optical system 24 is deviated upwards by a distance d, as indicated by the chain line, while moving along the rail 23. The launching mirror 6 is then also displaced upwards. As a result, the beam reflected by the disk 8 and led to the split-beam detector 15 deviates by the distance d. FIG. 8B shows how the beam is received by the split-beam detector 15 in this condition. Thus, an offset is generated in the track-deviation detection signal 17 as compared to the initial condition, as shown in FIG. 9B.
A similar offset is generated in the track-deviation detection signal 17 obtained from the split-light detector 15 when the movable optical system 24 is tipped. In that case, the beam reflected by the disk 8 and led to the split-beam detector 15 suffers a sideward deviation.
When an offset is thus generated in the track-deviation detection signal 17, the condensed spot 9 on the disk 8 cannot properly follow the guide grooves 8a, resulting in deterioration in the apparatus properties concerned with recording, reproducing, or erasing information.